Liquid developer for use in electrophotography

ABSTRACT

A liquid developer for use in electrophotography and being of the type in which a toner is dispersed in a carrier liquid, said carrier liquid consisting of an organic solvent having a high electric resistance of at least 1010 Omega cm. and a low dielectric constant of a maximum of 3, said toner being selected from the group of toners of the following types (a),(b),(c) and (d): A. Toners of the type in which pigment particles are coated, by the kneading technique, with a homopolymer of a monomer selected from the group consisting of polymeric monomers having an epoxy radical (-0-) and polymeric monomers having a carbinol radical, B. TONERS OF THE TYPE THAT SAID HOMOPOLYMER IS GRAFTCOPOLYMERIZED TO SAID PIGMENT PARTICLES, C. TONERS OF THE TYPE IN WHICH THE PIGMENT PARTICLES ARE COATED, BY THE KNEADING TECHNIQUE, WITH A COPOLYMER IN WHICH IS CONTAINED, BY COPOLYMERIZATION, A MONOMER SELECTED FROM THE AFORESAID GROUP OF POLYMERIC MONOMERS, AND D. TONERS OF THE TYPE IN WHICH SAID COPOLYMER IS GRAFTCOPOLYMERIZED TO THE PIGMENT PARTICLES.

United States Patent [72] inventors Hazime Machida;

Zenjiro Okuno, both of Tokyo, Japan [21 1 Appl. No. 748,237 [22] FiledJuly 29, 1968 [45] Patented Nov. 30, 1971 [73] Assignee Kabushiki KaishaRicoh Tokyo, Japan [32] Priority Aug. 4, 1967 [33] Japan [3l 1 42/49787[54] LIQUID DEVELOPER FOR USE IN ELECTROPHOTOGRAPHY 1 Claim, No Drawings52 1 us. CI 252/62.1, 260/33.6 EP, 260/4l R [5 l Int. Cl 003g 9/04 [50]Field of Search 252/62. l; 260/336; 1 17/37 LX [56] References CitedUNITED STATES PATENTS 2.899335 8/l959 Stranghan 252/62] 3078.23] 2/1963Metealfe et al. 252/62.l 3,438.904 252/621 4/1969 Wagner 3,503,88l3/l970 Shinoharaetal.

ABSTRACT: A liquid developer for use in electrophotography and being ofthe type in which a toner is dispersed in a carrier liquid, said carrierliquid consisting of an organic solvent having a high electricresistance of at least 10"Slcm. and a low dielectric constant of amaximum of 3, said toner being selected from the group of toners of thefollowing types n a. Toners ofthe type in which pigment particles arecoated, by the kneading technique. with a homopolymer of a monomerselected from the group consisting of polymeric monomers having an epoxyradical (-0-) and polymeric monomers having a carbinol radical,

b. toners of the type that said homopolymer is graftcopolymerized tosaid pigment particles,

c. toners ofthe type in which the pigment particles are coated by thekneading technique. with a copolymer in which is contained, bycopolymerization. a monomer selected from the aforesaid group ofpolymeric monomers, and

d. toners of the type in which said copolymer is graftcopolymerized tothe pigment particles.

LIQUID DEVELOPER FOR USE IN ELECTROPI-IOTOGRAPIIY BACKGROUND OF THEINVENTION 1. Field of the Invention v The present invention is concernedwith a liquid developer for use in electrophotography and of the type inwhich a toner is dispersed in a carrier liquid and the toner has apositive charge, said carrier liquid consisting of an organic solventhaving a relatively high electric resistance and a low dielectricconstant, said toner comprising pigment particles and a polymer selectedfrom the group consisting of homopolymers obtained from thepolymerization of monomers having specific radicals and the groupconsisting of copolymers obtained from the copolymerization of thosemonomers having specific radicals and other types of monomers, saidpolymer being either graft copolymerized to or coated on said pigmentparticles.

2. Description of the Prior Art Liquid developers for use inelectrophotography are, in general, prepared by kneading a mixture ofpigment particles, a polarity regulating agent, a dispersing agent and afixing agent, and dispersing the kneaded mixture in a solvent consistingof a paraffinic or an isoparaffinic hydrocarbon. More specifically, ithas been an indispensible requirement of the liquid developers of theprior art that the pigment particles which constitute the basic materialof the toner are coated with the aforesaid various agents, Le, apolarity regulating agent which is adapted to impress the toner with adistinct positive or negative electric charge to cause the deposition,by electrophoresis, of the toner onto the area of the electrostaticlatent image which is formed on the photoconductive layer of theelectrophotographic copying material, a fixing agent which is adapted toinsure the adhesion of the toner on the surface of said photoconductivelayer, and a dispersing agent which is adapted to have the toner stablydispersed in the carrier liquid. It has been the practice to use, as thepolarity regulating agents and the fixing agent, those materials such asnaturally occurring rosin, asphalt, synthetic resins and laurylmethacrylate-dimethylaminoethyl methacrylate copolymers, and also touse, jointly with the aforesaid materials, surface active agents, suchas metal soap, in order to impart a distinct polarity to the toner andto improve the dispersibility of the toner in the carrier liquid. It isalso known to affect the regulation of the polarity of the toners by thecombined use of two or more kinds of pigments. In spite of the pastefforts which have been paid to improve the ability of the toners asdesired, however, the following drawback of the toners has not beensuccessfully solved to date.

Specifically, the aforesaid drawback of the toners was represented bythe fact that, when the liquid developers of the prior art were storedfor an extended period of time or when the same liquid developers wereused repeatedly, it occurred that the polarity (meaning the positive ornegative electric charge) of the toners became indistinct so as toresult in a deterioration of the developing ability of the toners. Theevidence of the deterioration of the developing ability of the tonerswas detected by observing the lowered concentration of the toner in theliquid developer due to the repeated use thereof and caused by theelevated concentration of the additional agents such as the surfaceactive agent in the liquid developer, and in some cases, thedeterioration was detected by observing the sedimentation of theparticles of the toner in the bottom of the layer of the liquid. Whenthe liquid developer is converted to either one of the aforesaidconditions, not only will the copied image which is formed on thesurface of the photoconductive layer of the electrophotographic copyingmaterial lose clarity, but also there will occur a dark solid backgroundon the copying material and an insufficient fixation of the toner, andthe copy will not meet the practical requirements.

As has been described above, it has been extremely difficult for theliquid developers of the prior art to retain, for an extended period oftime, the initial'satisfactory developing ability which was possessed bythem at the time they were prepared.

SUMMARY OF THE INVENTION It is, therefore, the object of the presentinvention to provide a liquid developer for use in electrophotographywhich has overcome the aforesaid drawbacks of the liquid developers ofthe prior art and which, above all, is extremely stable and does notcause any deterioration of the developing ability during its repeateduse.

We, have discovered the following facts that, when a toner which isprepared either (1) by conducting the graft copolymerization of ahomopolymer of a polymeric monomer having an epoxy radical or a carbinolradical or the graft copolymerization of a copolymer which contains theaforesaid polymeric monomer to pigment particles, or (2) by coating thepigment particles with said polymeric monomer or with said copolymer bykneading the mixture of said pigment particles and said polymericmonomer or said copolymer, is dispersed in a carrier liquid consistingof a hydrocarbon, the resulting toner will bear a distinct positiveelectric charge, that the resulting toner exhibits a satisfactorydispersibility and a satisfactory developing ability without thecombined use of other additives, and further that these desirableproperties of the toner do not deteriorate after the storage of theliquid developer containing said toner for an extended period of timeand during the repeated use of such a liquid developer. It has beenfound also by us that the liquid developer containing the aforesaidgraft copolymerized toner and the liquid developer containing theaforesaid coated toner both possess a substantially equal developingability.

The present invention has been worked out based on the aforesaidknowledge.

The carrier liquid which is used with the toner in the present inventioncan be the carrier liquids which have been employed in general inelectrophotography. It is to be noted, however, that, since the polarityregulating agents (resins) and the other additives which were used inthe liquid developers of the prior art were selected, with preference,from among those having a low solubility in the carrier liquids, it isdesirable, in the present invention, to use those polymeric monomershaving an epoxy radical or a carbinol radical, in the form of acopolymer containing such a substance as lauryl methacrylate as thecounterpart constituent of the copolymer. Those toners which areprepared either by graft copolymerizing the aforesaid copolymers topigment particles or by coating the pigment particles with suchcopolymers by kneading the mixture of the pigment particles and thecopolymers, exhibit a much better dispersibility in the aforesaidcarrier liquids, as compared with the dispersibility which is exhibitedby the toners prepared either by graft-copolymerizing homopolymers topigment particles or by coating the pigment particles with homopolymersby kneading the mixture of these two constituent materials.

As the polymeric monomers having an epoxy radical which are applicableto the present invention, those monomers such as glycidyl methacrylate,2,3-epoxypropyl acrylate, allyl-2,3- epoxypropyl maleate, andallyl-2,3-epoxybutyrate are suitable. Also, as the polymeric monomershaving a carbinol radical which are applicable to the present invention,those monomers such as hydroxyethyl methacrylate, allylalcohol, 2-ethyl-2-hydroxylethyl-aminoethyl acrylate, diethyl-vinylethynylcarbinol, B-hydroxylbutyl acrylate, methyl-phenylvinylethynyl carbinol,methyl-ethyl-vinylethynyl carbinol, ndipropyl-vinylethynyl carbinol andn-propyl-vinylethynyl carbinol are suitable. Furthermore, as themonomers which are copolymerized with one of the aforesaid polymericmonomers, those monomers including alkyl methacrylate such as butylmethacrylate and lauryl methacrylate, styrene, styrene derivatives suchas aminostyrene, hydroxystyrene, chlorostyrene and methylstyrene;acrylamide, acrylamide derivatives and acrylic acid derivatives such asdibutylacrylamide, dimethyl-acrylamide and a-methyl-acrylamide; dienederivatives such as 2-n-butylbutadiene, 1,3-cyclohexadiene and2-n-amylbutadiene-l,3, dibasic acid such as anhydrous maleic acid andcrotonic acid; and acrylonitrile derivatives and methaerylonitrilederivatives such as acrylonitrile and aethylaerylonitrile, are suitable.

in case a copolymer of the aforesaid type is used with the pigmentparticles, the object of the present invention will be attained by theemployment of a two or more component copolymer in which is contained apolymeric monomer having the aforesaid specific radical in amount atleast percent by mole, as will be understood clearly from theembodiments of the present invention which will be described later.

As has been described, the present invention contemplates the provisionof a liquid developer for use in electrophotography which is of anextremely simple structure such that a toner which is comprised of acombination of only pigment particles and a polymer is dispersed in acarrier liquid. It is to be noted, however, that, in spite of such asimple structure of the liquid developer, the toner bears a verydistinct polarity and also that it has a satisfactory dispersibility inthe carrier liquid and that, accordingly, when the liquid developer ofthe present invention is used in the developing process, and the copiedimages obtained are satisfactory in both the clarity and the fixation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 To 0.2 mole ofglycidyl methacrylate, 0.2 mole of butyl methacrylate, 0.41 mole ofstyrene and 0.3 mole of lauryl methacrylate was added 0.005 mole ofazobisisobutyronitrile, and there further was added 1 mole of toluene.The resulting mixture was allowed to react in hydrocarbon, nitrogenatmosphere at 90 C. for 9 hours, and thus, a highly viscous copolymerwas prepared. To 7 parts by weight of said copolymer were added 1 partby weight of carbon-black (produced under the trade name of AsahiXC-550, by Asahi Carbon, Ltd. ofJapan) and 32 parts by weight of NaphthaNo. 6 (trade name of a petroleum hydrocarbon i.e., an aliphatichydrocarbon solvent containing weight percent aromatics, flash point 40C., specific gravity 0.789, produced by ESSO Standard Oil Company ofU.S.A.). The resulting mixture was kneaded in a ball mill for 8 hours,and as a result, a highly concentrated toner was prepared. Four grams ofthe concentrated toner thus obtained was introduced in 1 liter of lsoparH (trade name of an isoparafiinic hydrocarbon, flash point 123 F.,Kauri-Butanol value 27, specific gravity 0.7571, produced by ESSOStandard Oil Company of U.S.A.). Thus, a liquid developer for use inelectrophotography of the type in which the toner consisted of thepigment particles coated with the aforesaid copolymer and having apolarity regulated to the positive charge was dispersed in the carrierliquid was obtained.

Using a commercially available electrophotographic copying material, anelectrostatic latent image was given, this copying material according toa known technique, and then the resulting copying material was developedby immersing the latter in the liquid developer of the presentinvention. The result was that a very clear positive copied image wasformed. Also, the liquid developer of the present invention was left tostand for more than 6 months and furthermore, the liquid developer thusobtained was subjected to repeated use for such an extended period oftime. However, neither of these liquid developers of the presentinvention showed any appreciable deterioration of their developingabilities.

EXAMPLE 2 To 7 parts by weight ofa monomer-constituting liquidconsisting of:

styrene 0.2 mole lauryl methacrylate 0.2 mole hydruxyethyl methacrylate0.3 mole 0.005 mole 1.0 mole was added 1 part by weight of carbon-black(produced under the trade name of Statexdeness by Columbian CarbonCompany of U.S.A.). In a manner similar to that described in example l,the resulting mixture was allowed to react, and thus, agraft-copolymeric toner wherein the copolymer is united with the pigmentparticles was prepared. To 1 part by weight of this toner was added 4parts by weight of Naphtha No. 6, and the resulting mixture was kneadedin a ball mill for 8 hours to disperse the components thoroughlyrelative to each other. As a result, a highly concentrated toner wasobtained. Four grams of this concentrated toner was dispersed in 1 literof Naphtha No. 6, and thus a liquid developer for use inelectrophotography wherein the polarity of the toner was regulated tothe positive charge was obtained. Using this liquid developer, anelectrophotographic copying material which had been given anelectrostatic latent image by a known technique was developed. Theresult was that a very clear positive copied image was formed. Thisliquid developer was left to stand or subjected to repeated use for aperiod of more than 6 months, but it showed no appreciable deteriorationof its developing ability in each of these tests.

EXAMPLE 2 A copolymer was prepared, in a manner similar to thatdescribed in example 1, by the use of the copolymer-constituting liquidhaving the following composition:

lauryl methaerylate 1.0 mile hydroxyethyl methacrylate 0.6 moleazobisisohutyronitrilc 0.005 mole toluene 1.0 mole By blending thiscopolymer with carbon-black (produced under the trade name of MitsubishiNo. 44, by Mitsubishi Chemical Industries, Ltd. of Japan) and also withNaphtha No. 6, the resulting mixture was kneaded thoroughly in a ballmill. Thus, a highly concentrated toner was prepared. Four grams of thisconcentrated toner was dispersed in 1 liter of Naphtha No. 6, and as aresult, a liquid developer for use in electrophotography wherein thepolarity of the toner was regulated to the positive charge was obtained.The liquid developer thus obtained was applied to the development of theelectrophotographic copying material which was similar to that describedin example 1. The result was that a very clear positive copied image wasformed. The liquid developer thus obtained was left to stand orsubjected to repeated use for a period of more than 4 months, but noappreciable deterioration ofthe developing ability was noted in each ofthese tests.

EXAMPLE 4 A copolymer was prepared, in a manner similar to thatdescribed in example 1, by the use ofa copolymer-constituting liquidhaving the following composition:

lauryl methacrylute 0.6 mole allyl alcohol 0.4 mole toluene 1.0 moleazuhisisobulyronitrile 0.005 mole 7 parts by Weight of this copolymer, 1part by weight of Spirit Black (a product of Orient Chemical Industries,Ltd. of Japan) and 32 mole parts by weight oflsopar H were kneadedtogether in a ball mill for 8 hours, and thus, a highly concentratedtoner was prepared. Four grams of this concentrated toner was dispersedin 1 liter of lsopar H, and as a result, a liquid developer for use inelectrophotography was obtained. The liquid developer thus obtained wasapplied to the development of an electrophotographic copying materialsimilar to that described in example 1. The result was that a very clearpositive copied image was formed. This liquid developer was left tostand or subjected to repeated use for a period of more than 4 months,but no deterioration of the developing ability was noted in each ofthese tests.

What is claimed is:

1. A liquid developer for use in electrophotography consistingessentially of a liquid hydrocarbon having a high electric resistance ofat least lQ-cm. and a low dielectric constant of a maximum of 3 and atoner dispersed in said liquid hydrocarbon, said toner consistingessentially of particles of a pigment selected from the group consistingof Carbon Black (C.l. No. 77,266) and Spirit Black (C.l. No. 50,440) anda coating-layer coated on said particles, said coating-layer consistingessentially of a polymer selected from the group consisting of:

a. homopolymers of a monomer selected from the group consisting ofglycidyl methacrylate, 2,3-epoxypropyl acrylate, allyl-2,3-epoxypropylmaleate, allyl-2,3-epoxybutyrate, hydroxyethyl methacrylate,allylalcohol, 2- ethyl-2-hydroxyiethylaminoethyl acrylate,diethylvinylethynyi carbinol, ,B-hydroxybutyl acrylate,methylphenyl-vinylethynyl carbinol, methyl-ethyl-vinylethynyl carbinol,n-dipropyl-vinylethynyl carbinol and n-propylvinylethynyl carbinol; and

b. copolymers of at least 15 mol percent of a monomer selected from saidgroup of monomers named in (a) and the balance consisting of at leastone monomer selected from the group consisting of butyl methacrylate,lauryl methacrylate, styrene, aminostyrene, hydroxystyrene,chlorostyrene, methylstyrene, acrylamide, dibutylacrylamide,dimethylacrylamide, a-methyl-acrylamide, 2-nbutylbutadiene,1,3-cyclohexadiene, 2-n-amylbutadiene- 1,3, anhydrous maleic acid,crotonic acid, acrylonitrile and a-ethyl acrylonitrile.

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